(1) Field of the Invention
The present invention relates to an enciphered communication system, and more particularly, to a system for enciphered communications in a data communication system wherein a plurality of cipher keys are used for enciphering data and are switched as required.
(2) Description of the Related Art
Conventionally, measures are taken to encipher secret data before transmission so that the contents of the data may be unintelligible even if intercepted during transmission via a communication line. Enciphering methods include DES (Data Encryption Standard) and FEAL (Fast data Encipherment ALgorithm) using secret or publicly open cipher keys, and an identical cipher key is used at transmitting and receiving sides to accomplish normal data communications.
FIG. 16 illustrates the configuration of a conventional enciphered communication system. As shown in FIG. 16, the conventional enciphered communication system comprises a transmitter 1 for enciphering and transmitting data, and a receiver 2 for receiving and deciphering the enciphered data. A transmission line 3 connecting the transmitter 1 and the receiver 2 is not particularly limited, and may be of a wired type such as a telephone line or a wireless type as used in communications utilizing a satellite. FIG. 16 illustrates only the enciphering/deciphering-related parts of the transmitter 1 and the receiver 2.
The transmitter 1 includes a data enciphering section 11 for enciphering input data by means of a cipher key ks, a cipher key table 12 for converting a specified number N(ks) to a cipher key ks, a cipher key number setting section 13 for setting a number N(ks) by which a cipher key ks in the cipher key table 12 is selected to be used for enciphering the input data, a cipher key number sending section 14 for sending the cipher key number N(ks) selected by the cipher key number setting section 13 to the receiver 2, and a multiplexing section 15 for multiplexing the enciphered data and the cipher key number N(ks).
The receiver 2 includes a separating section 21 for receiving data transmitted via the transmission line 3 and separating the received data into enciphered data and a cipher key number, a cipher key number receiving section 22 for receiving the separated cipher key number, a cipher key table 23 having the same contents as the cipher key table 12 of the transmitter 1, and a data deciphering section 24 for deciphering the separated enciphered data by means of a cipher key ks specified by the cipher key number N(ks)'.
Thus, the transmitter 1 and the receiver 2 have identical cipher key tables 12 and 23, respectively, each having a plurality of cipher keys, and only the cipher key number corresponding to the cipher key which is used to encipher the transmit data is transmitted to the receiver 2. The receiving side decodes the received data to acquire the cipher key number, and obtains the same cipher key as used at the transmitting side from the cipher key number to decipher the enciphered data. This arrangement permits the cipher key number to be changed as needed, and also ensures high-secrecy transmission because the cipher key itself is not transmitted.
FIG. 17 schematically illustrates data on a transmission line of the conventional enciphered communication system. As shown in FIG. 17, the data transmitted onto the transmission line 3 from the transmitter 1 consists of input data D enciphered by the data enciphering section 11, the cipher key number N(ks) output from the cipher key number sending section 14 and multiplexed at the multiplexing section 15, and frame synchronizing signals F. The cipher key number N(ks) is usually transmitted only when it has been changed; in some cases, the cipher key number is transmitted at the start of data communication or at regular intervals, or is transmitted with each of frames.
In the conventional enciphered communication system having a plurality of cipher keys, information about the cipher key number is transmitted from the transmitter to the receiver. Thus, if one of data items relating to the cipher key number, among the information transmitted when the cipher key number is changed, is accidentally in error, for example, all the other received data becomes erroneous because the receiver deciphers the received data by means of the cipher key corresponding to the previously notified cipher key number, making normal deciphering impossible. In such cases, moreover, received data continues to become erroneous until the cipher key number is correctly notified.
In the case of a two-way communication system, retransmission of the cipher key number can be requested upon detection of discrepancy between the cipher key numbers. However, in a one-way communication system such as a broadcasting system which allows no such retransmission request, received data continues to become erroneous until the cipher key number is correctly notified.